1. Technical Field
The present invention relates generally to the recovery of offshore hydrocarbons, and more particularly, to an offshore system and method for storing a continuous length of jointed tubular conduit and for tripping the continuous length of jointed tubular conduit down to or up from a subsurface well utilized in the production of hydrocarbons.
2. Background Information
Offshore hydrocarbon exploration is presently yielding an increasing number of reserves in deepwater regions, which are generally defined as offshore regions having water depths greater than about 600 meters. There are, however, a myriad of operational and economic complexities associated with the exploitation of deepwater hydrocarbon reserves that conventional drilling and production technologies do not adequately address. For example, many drilling, production, and well servicing applications require placement of a tubular conduit, termed a riser, that extends from the operating deck of a fixed or floating surface structure to the marine floor. The riser serves as an access conduit through the water column from the operating deck to the subsurface wellhead for the support and protection of drillstrings, production strings, service tools, or the like.
Currently, most risers are constructed from high-strength steel. Although steel risers generally perform satisfactorily in shallow or moderate water depths, the use of steel risers in deepwater operations, where water depths can exceed 1000 meters, becomes problematic. Because deepwater operations require risers of extraordinary length to reach the marine floor, the excessive weight of such long length risers becomes a limiting operational parameter. The problem is particularly acute in drilling operations where floating vessels, such as drill ships or semisubmersibles, are commonly used to support the drilling riser and the associated drillstring. Extremely large drilling vessels are required to adequately support the weight of conventional steel drilling risers and drillstrings in deep water. However, in many cases, the use of large-scale drilling vessels is economically impractical because such vessels are costly to build and maintain and, therefore of limited availability.
In response to this problem, drilling risers have been developed from alternate materials that have an equivalent or greater strength than steel, yet are considerably lighter, thereby significantly reducing the weight load on the drilling vessel in deepwater applications. Such alternate riser materials are generically termed "composites," of which particular types are described in U.S. Pat. No. 4,634,314 and European Patent Application EP 0 244 048 A2. In addition to having characteristics of low density and high mechanical strength, composites also have favorable corrosion and fatigue resistant properties, rendering them potentially desirable even in shallower water applications. Other composites useful in the fabrication of risers having utility in deepwater applications are described in a number of literature references including Sparks et al., "High Performance Composite Tubes For Deepwater Risers"; Falcimaigne, "High Performance Composite Pipes For Deep Water Multiline Production Risers"; and Brookes et al., "Research & Development In Riser Systems". These references emphasize the particular effectiveness of risers fabricated from composites for use in association with floating vessels, such as semisubmersibles, because composites are generally more compliant than their steel counterparts and, thus, more adaptable to movement of the floating vessel due to motion of the body of water in which the floating vessel resides.
The advancement of composites for the fabrication of relatively lightweight, high-strength risers has made inroads toward eradicating a primary technological impediment to the exploitation of deepwater hydrocarbon reserves. Nevertheless, the cost of conventional methods for on-site storage, installation and retrieval of tubular conduit for deepwater applications, whether fabricated from steel or composites, remains a significant impediment to the exploitation of deepwater hydrocarbon reserves. For example, drilling risers are conventionally stored on-site by stacking the riser as disassembled straight sections of tubular conduit on the deck of the support vessel. The riser is subsequently installed by tripping it from the surface to the wellhead in a section-by-section process termed stalking, wherein each new section of the riser is positioned on its end atop the deck of the surface structure and assembled to the end of the preceding section extending into the water. The procedure is repeated one section at a time until the assembled riser extends substantially the entire distance from the deck to the wellhead. If it is desired to retrieve the riser, it is tripped back to the surface in the reverse procedure, disassembling the riser section-by-section as the riser is drawn up from the wellhead to the deck until the entire riser has been removed from the water and disassembled on the deck. It is apparent that tripping a long length riser in this manner for deepwater applications is extremely time consuming due to the large number of sections making up the riser, each of which must be individually handled. Any activity that increases time consumption translates directly to added cost because the drilling rig, crew and equipment must remain on-site during the tripping operation and drilling activity must be suspended while the riser is being tripped. In addition, the space required to stack the disassembled riser sections during on-site storage may exceed the available storage space of the drilling vessel, thereby requiring a stand-by support vessel for storage of sufficient riser sections to make up the needed length of assembled riser. The required presence of a support vessel unduly adds to the cost of the drilling operation. Accordingly, the present invention recognizes a need for less time consuming and more cost-effective means for tripping a tubular conduit between the deck of a surface structure and a subsurface wellhead and further recognizes the need for a less space-intensive and more cost-effective means for storing a tubular conduit at an offshore site.
A number of alternate methods, other than the above-described stalking method, are known for storing and deploying tubular conduit in marine environments. U.S. Pat. No. 3,777,827 discloses an apparatus for storage of a continuous flexible drill pipe in an on-site basket surrounding the drilling rig and for deployment of the drill pipe in a subterranean or subsea borehole while supplying a drilling fluid to the drill pipe. U.S. Pat. No. 4,917,540 discloses an apparatus for storing a rigid pipeline on a shipboard reel and laying the pipeline along the ocean floor while straightening and adjusting the tension thereof. U.S. Pat. No. 4,730,677 discloses an apparatus for storing a flexible riser on a powered reel of a support ship and deploying the riser from the ship to the wellhead of a completed wellbore for maintenance or servicing of the wellbore with either wireline or through the flow line tools. None of the above-recited apparatuses for storing and deploying tubular conduit, however, relates to the specific performance requirements for storing, installing or retrieving continuous lengths of jointed drilling risers in deepwater environments. In particular, the prior art neither recognizes nor addresses the unique problems attributable to the magnitude of an offshore drilling riser storage and tripping operation with respect to the extreme length required of the riser for deepwater applications. Nor does the prior art recognize or address the unique problems attributable to the fact that offshore drilling risers typically have external lines such as choke and kill lines extending coaxially with the riser that require special attention during the tripping operation to avoid damaging the lines.
As such, it is an object of the present invention to provide an apparatus and method for storing and deploying a tubular conduit offshore in a time-efficient and cost-effective manner. More particularly, it is an object of the present invention to provide an apparatus and method for storing a continuous jointed drilling riser or a continuous jointed drillstring on the deck of a floating drilling vessel and for tripping the drilling riser or drillstring between the deck and a subsurface wellhead in a time-efficient and cost-effective manner. It is another object of the present invention to provide an apparatus and method for storing and tripping a tubular conduit having specific utility to semisubmersibles. It is still another object of the present invention to provide an apparatus and method for storing and tripping a tubular conduit having specific utility to a deepwater marine environment. It is yet another object of the present invention to provide an apparatus and method for tripping a continuous jointed drilling riser within a marine environment, wherein the riser can be broken at its joints to remove a section for service or replacement thereof. It is yet a further object of the present invention to provide a time-efficient and cost-effective apparatus and method for storing and tripping a continuous jointed drilling riser within a marine environment, wherein the riser has one or more external lines associated therewith. It is a still further object of the present invention to provide a continuous jointed drillstring that is compatible with the apparatus and method for storage and deployment thereof. These objects and others are achieved in accordance with the invention described hereafter.